Quantum Mechanics is strange. Particles seem to exist in states of uncertainty, an uncertainty that collapses when we try and take a look at what they are doing. This oddness has led to all kinds of ways of looking at quantum mechanics to try and understand what is the reality 'behind the scenes' of it all.
The most commonly described way of looking at quantum mechanics is called the 'Copenhagen Interpretation', which states that there is something that happens when a quantum situation is somehow 'observed' which forces the state into a defined value as against uncertainty. This interpretation has led to all kinds of nonsense, such as the idea that human minds are necessary to act as observers, but it's still widely used as a way of dealing with quantum situations.
Another way of looking at quantum mechanics which is supposed to remove the failings of the Copenhagen Interpretation is called the "Many Worlds Interpretation". This interpretation takes the view that quantum uncertainty doesn't collapse into certainty in the way we might think it does. Instead, all quantum possibilities actually happen, but because we are involved in the quantum world too, each of us is split into copies, one of which will see each of the the possibilities.
"Many Worlds" is quite popular, and often thought to be more reasonable than the idea of quantum probability collapse. I'm going to try and show that this isn't the case.
One objection to "Many Worlds" is that it results in an unimaginable number of parallel universe, and that is though to be excessive. This isn't really a sensible objection, because an hypothesis should be supported because of the simplicity of its premises and logic, not the number of outcomes.
I don't object to "Many Worlds" because of the number of parallel worlds. My objection is quite different. My objection is that it doesn't add any explanatory value.
Consider a quantum state which can be one of two values: V1, and V2. This might be the polarization of a photon or the spin of an electron, for example. Now imagine a person, A, who does an experiment to determine the quantum state. According to Many Worlds the result of the experiment is that there are two copies of A, A1 and A2, who see two different values: A1 sees V1 and A2 sees V2.
Now, suppose A1 asks the question 'why do I see value V1?'. The Copenhagen Interpretation says "the quantum state collapsed and randomly selected V1". The Many Worlds Interpretation says "don't worry - there is another you, A2, who sees V2". Effectively, what Many Worlds says is "there is another you, and somehow it was randomly decided that between you and other you, one would see V1 and the other would see V2". So, each copy of A could justifiably ask why they were not seeing what the other copy was seeing, and the answer to both would be "it's random".
The Many Worlds Interpretation gives the impression that there is no collapse of quantum states, but that isn't true. Each person will see a history of quantum observations, each of which could have given an alternative result. That there are other copies of that person does not add any explanatory value when it comes to trying to explain why one person sees a particular history.
Many Worlds is excessive, not because it introduces parallel words per se, but because those parallel worlds don't add any explanatory value. It's more reasonable to say that there are many alternatives, but only one actually exists.